Nitrogen oxide storage catalytic converters (also referred to as NOx storage cats or lean NOx traps, LNT) are used for the temporary adsorption of nitrogen oxides from the exhaust gas of internal combustion engines. In addition, they perform the functions of oxidative aftertreatment of carbon monoxide (CO) and hydrocarbons (HC). Nitrogen oxides which form during lean-burn operation of an internal combustion engine can be stored in an LNT; for this purpose, the LNT oxidizes the nitrogen monoxide (NO) contained in the lean exhaust gas to form nitrogen dioxide (NO2), and subsequently stores this in the form of nitrates. Examples of adsorbents that are used in the coating of the LNT are barium oxide and/or other oxides.
Once the storage capacity of the LNT is exhausted, the LNT must be regenerated. In a regeneration event (purge), rich, substoichiometric exhaust gas conditions are provided, e.g. by operating the internal combustion engine with a corresponding fuel/air mixture; during this process, the stored nitrogen oxides are desorbed again and are reduced to nitrogen over catalytically active components of the LNT with the aid of the components in the rich exhaust gas (CO, HC). In addition to a purge brought about solely for regeneration, the LNT is, of course, also regenerated when the exhaust gas becomes substoichiometric by reason of a power demand of the internal combustion engine, for example.
In the LNT, the stored nitrates furthermore react with molecular hydrogen, which is produced by incomplete combustion of the fuel and also by reactions in the LNT under rich exhaust gas conditions, as a result of which ammonia is also produced during regeneration. This ammonia can be utilized by storing it downstream in a catalytic converter for selective catalytic reduction (SCR). The stored ammonia is used in the SCR to reduce nitrogen oxides to nitrogen under lean exhaust gas conditions. To enable the SCR catalytic converter to have a higher storage capacity, it is advantageous to install it sufficiently far downstream to ensure that optimum operating temperatures are obtained for it. The corresponding temperature range is a function of the specific SCR coating and is known to a person skilled in the art.
Among the factors which limit the storage capacity of an LNT is the temperature of the exhaust gas. Modern LNTs can store nitrogen oxides with varying efficiency in a temperature range of 250-550° C. Moreover, the storage capacity can be restricted by the space velocity of the exhaust gas. When the internal combustion engine is operated under a high load, e.g. during an acceleration event, high exhaust gas temperatures and mass flows are achieved, and these can exceed the technological limits of the LNT, severely reducing the nitrogen oxide storage capacity of the LNT owing to the gas temperature and space velocity. Under these conditions, nitrogen oxides cannot be stored in the LNT. It is possible to counteract the escape of nitrogen oxides under high loads by switching between different combustion modes, depending on the engine load, depending on the saturation of catalysts, and depending on the exhaust gas temperature. These modes include states involving lean exhaust gas and a state involving rich, substoichiometric exhaust gas (see applications DE 10 2016 210 897.2 and DE 10 2016 210 899.9).
However, the inventors herein have recognized an issue with the above approach. In exhaust systems with two LNTs, when the downstream LNT demands purge, excess ammonia may be generated in the upstream LNT. Further, when post-injection of fuel to the engine is used to generate the rich exhaust for purge of the downstream LNT, more fuel than actually necessary for carrying out the purge may be supplied, in part because the downstream LNT may flow exhaust gas at a lower mass flow rate than the upstream LNT.
Accordingly, the inventors herein provide systems and methods to at least partly address the above issues. In one example, an arrangement for a vehicle including an internal combustion engine with an exhaust tract includes a low-pressure exhaust-gas recirculation line branching off from the exhaust tract downstream of a turbine and an exhaust gas aftertreatment system arranged in the exhaust tract. The exhaust gas aftertreatment system includes a first nitrogen oxide storage catalytic converter (also referred to as a first LNT), a second nitrogen oxide storage catalytic converter arranged downstream of the first nitrogen oxide storage catalytic converter (also referred to as a second LNT), a particle filter arranged downstream of the first nitrogen oxide storage catalytic converter and upstream of the second nitrogen oxide storage catalytic converter, and a first feed device for introducing fuel into the exhaust tract and arranged downstream of the branching off of the exhaust gas recirculation line and upstream of the second nitrogen oxide storage catalytic converter.
A first aspect of the disclosure relates to an arrangement of an internal combustion engine with an exhaust tract, from which at least one low-pressure exhaust gas recirculation line branches off and in which an exhaust gas aftertreatment system is arranged. The exhaust gas aftertreatment system comprises at least one first nitrogen oxide storage catalytic converter (LNT), at least one second LNT, which is arranged downstream of the first LNT, at least one particle filter, which is arranged downstream of the first LNT, and at least one first feed device for introducing fuel into the exhaust tract, which device is arranged downstream of the branching off of the exhaust gas recirculation line and upstream of the second LNT.
The arrangement according to the disclosure is advantageous because it allows external fuel injection upstream of the second LNT. As a result, it is sufficient for rich exhaust gas to be made available for regeneration or a substoichiometric mode by post-injection only for the first LNT, while the exhaust gas for regeneration or a sub stoichiometric mode of the second LNT is provided by external introduction of fuel into the exhaust tract. Therefore, a reduced quantity of additionally injected fuel (in comparison with operation without external fuel injection) is required for the first LNT. It is thereby advantageously ensured that less fuel is carried into the engine oil, which is less severely diluted as a result, leading to less severe impairment of the lubricating properties of the oil. Furthermore, restrictions in respect of the material temperature limits of a turbocharger turbine arranged in the exhaust tract are of lesser weight if the conditions for a substoichiometric mode are brought about by introducing fuel into the exhaust tract. In addition, less fuel is required for the substoichiometric mode or regeneration of the second LNT than in the case of the first LNT since the exhaust gas mass flow through the second LNT is lower than that through the first LNT. Moreover, the first LNT can remain in the lean exhaust gas during a substoichiometric mode or regeneration of the second LNT. Furthermore, it is not necessary to reduce the oxygen quantity stored in the first LNT in order to achieve rich exhaust gas conditions in the second LNT, and this likewise has a favorable effect on fuel consumption. Moreover, the ammonia which can form in the first LNT under conditions of high load and rich exhaust gas can advantageously be used for further reduction of the nitrogen oxides in the second LNT.
In this case, rich-mixture operation of the internal combustion engine may be triggered, especially under conditions with a high load and high exhaust gas temperatures resulting therefrom. Under these conditions, the first LNT no longer operates as a storage catalytic converter but immediately converts the nitrogen oxides present in the exhaust gas to nitrogen with the aid of the reducing agents (carbon monoxide and hydrocarbons) which are likewise present in the exhaust gas. In this way, nitrogen oxides are advantageously removed from the exhaust gas emerging from the internal combustion engine under high-load conditions. Moreover, the level of enrichment in the exhaust gas may be adjusted so that, under these conditions, ammonia is formed over the catalytically active components of the first LNT by the reaction of hydrogen with nitrogen oxides as soon as previously stored oxygen has been removed from the first LNT. In an example, this ammonia can be used for further, downstream reduction of the nitrogen oxides with the aid of the second LNT.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.